Magnetic energy storage device

ABSTRACT

A superconducting magnetic energy storage (SMES) device including a first coil made of superconducting material, a cooling mechanism for cooling the first coil to superconducting temperatures, a second coil inductively coupled to the first coil for inputting emergy to, and/or outputting energy from, the first coil, and a switch for switching the first coil between a superconducting condition and a non-superconducting condition. The first coil is arranged as a closed loop electric circuit having no connecting device mechanically connected to it for inputting or outputting energy. The switch includes a third coil for the application or removal of a magnetic field for switching the first coil between its non-superconducting and superconducting conditions. A method inputs energy to and/or outputs energy from the first coil and a power supply system utilizes the device and method.

TECHNICAL FIELD

This invention relates to a superconducting magnetic energy storage(SMES) device of the kind comprising a closed first coil means made ofsuperconducting material, cooling means for cooling the first coil tosuperconducting temperatures, second coil means inductively coupled tothe first coil means for inputting energy to, and/or outputting energyfrom the first coil means, and switching means for switching the firstcoil means between a superconducting condition and a non-superconductingcondition. Although the invention primarily relates to a SMES device inwhich the first coil means comprises a high-transition temperaturesuperconducting (HTS) material it also relates to a SMES device in whichthe first coil means comprises a low-transition temperaturesuperconducting (LTS) material. The invention also relates to a methodof inputting and outputting energy to and from a network and to anetwork protection system.

The invention has application in maintaining power quality against shortterm power or voltage reductions, in serving as a storage system inorder to smooth peak loads, in providing an “uninterruptible” powersupply in a grid or even in a supply system for an industrial processwhere power disturbances are very expensive and damaging, such as, forexample, a paper mill or a steel mill. The invention may also haveapplication as an energy storage device in vehicles, including ships,aeroplanes and other types of vehicles such as electric and hybridelectric/combustion powered cars, trucks and buses.

BACKGROUND OF THE INVENTION

In a SMES device, energy in form of a magnetic field can be stored in acoil made of superconducting material where a superconducting currentcirculates. The stored energy is ½.L.I² where L is the inductance and Iis the current circulating in the coil. Since the coil issuperconducting, very small losses are present and the storage time isvery long. When energy is required, the current can be redirected andthe energy stored in the coil is transferred to work in the coil. Most,if not all, of the present day solutions for charging and discharging aSMES are based on redirecting the current into or out of the coil usingswitches. Generally the switches are mechanical or solid state or acombination of both and can either be placed in the cooling medium thatcools the superconducting coil or placed outside the cooling medium,e.g. at room temperature. However, in both cases, the redirection of thecurrent is based on the principle of breaking the superconductingcircuit to force the current to flow in a path through the load. Inparticular, in such known devices, current leads are present between thecooled superconducting coil and the outside in order to input energy to,or output energy from, the superconducting coil in the form of anelectrical current.

The design of such feed-through current-carrying leads is one of themain difficulties when constructing a SMES device. At some point of thefeed-through there must be a change from a superconducting conductor toa good room-temperature electrical conductor. However, a good electricalconductor is generally also a very good heat conductor, which makes thethermal isolation of the cold space difficult.

Another drawback with using conventional switches is that the switchesalways exhibit resistive losses since it is practically impossible toconstruct a purely superconducting switch. Therefore, conventional SMESdevices will suffer from losses even in stand-by mode, which is aserious drawback for long time storage devices.

It is of course known to transfer energy between coils using induction.For example this concept is used with transformers. However, inductiveenergy transfer is only possible with time varying currents. In bothU.S. Pat. No. 4,939,444 and U.S. Pat. No. 5,682,304, SMES apparatusesare disclosed in which coils are inductively coupled together. Theseknown apparatus use oscillation circuits to create oscillating currents.The authors of these known specifications have correctly realised thatthe superconducting properties of a coil are destroyed if the magneticfield is too high. They refer to the possibility of storing a largeamount of energy in the coil since the magnetic field will destroy itssuperconducting properties. The solution to the problem suggested bythese authors is to wind a pair of coils in opposite directions in orderto cancel out the magnetic field. However, it is claimed that, since themagnetic field is cancelled out, the current flowing in thesuperconducting coil arrangement can be increased substantially. It isbelieved, however, that such a coil arrangement will not store anyenergy although the current can be increased. This is because the energyis “in” the magnetic field itself. With no magnetic field no magneticenergy is present.

U.S. Pat. No. 5,682,304 does disclose a number of inductive couplingsbetween the coils. However, the couplings between coils 9 and 12 andbetween coils 22 and 24 are only present in order to cancel out themagnetic field. Moreover, in order to achieve an oscillating circuitformed by components 30 and 33, the coupling factor between coil 36 andcoil 30 must not be too good. With perfect coupling the inductance inthe LC-circuit is zero. However, with a coupling factor less than unityan energy transfer may be possible since the leak inductance is largerthan zero, although this is not what is described.

One aim of the present invention is to provide an improved SMES deviceof the kind referred to in which there are no mechanical connectionsconnected to the superconducting first coil means for inputting energyto and/or outputting energy from the first coil means.

Another aim of the present invention is to enable energy to be inputtedto or partially extracted from stored energy of superconducting coilmeans.

According to one aspect of the present invention there is provided asuperconducting magnetic energy storage (SMES) device comprising a firstcoil means made of superconducting material, cooling means for coolingthe first coil to superconducting temperatures, second coil meansinductively coupled to the first coil means for inputting energy to,and/or outputting energy from the first coil means, and switching meansfor switching the first coil means between a superconducting conditionand a non-superconducting condition, characterised in that the firstcoil means is arranged as a closed loop electric circuit having noconnecting means mechanically connected thereto for inputting oroutputting energy thereto, and in that the switching means comprises athird coil means for the application or removal of a magnetic field forswitching the first coil means between its non-superconducting andsuperconducting conditions.

The invention is based on the concept of transferring energy between thesuperconducting (cryogenic) first coil means to the normally conductingsecond coil means at an elevated (relative to cryogenic) temperature,e.g. at room temperature, without using an oscillating circuit. Therequired time varying current is for inductively transferring energy isobtained by making the superconducting material of the first coil meansnormal-conducting with a large enough magnetic field from a the thirdcoil means, preferably arranged perpendicular to the other two coilmeans. As long as this quench-field is applied the current in the firstcoil means decays. According to Faraday's law of induction the secondcoil means tries to oppose this change by inducing a current in the samedirection. This current is used to provide a load with power. Inpractice a capacitor is charged to a desired voltage which, in turn, isdischarged in a controlled way over a load. (If the load requires anac-current there must be a dc-ac converter between the capacitor and theload.)

The present invention is based on experimental results proving that itis possible to partially or completely discharge a closed-loopsuperconductor in the manner described above. This opens thepossibilities to isolate the cryogenic parts from the parts that can ormust be at an elevated temperature, e.g. at room temperature, in a SMESapplication. The main consequence of this is that the heat leakage intothe cryogenic parts can be minimised. Furthermore, a bulk cylinder ofthe superconducting material can be used as a one-turn first coil means.Since the first coil does not have switches or other componentsphysically attached to it, the resistive losses are minimised.

Although not essential, it is preferred that the first coil meanscomprises a single turn, e.g. in the form of a cylinder. Alternatively,for example, the first coil means may be of other shapes, e.g. oftoroidal form.

Preferably the switching means further comprises control means forcontrolling the current supplied to the third coil means. In this waythe amount of quenching of the superconducting first coil means can becontrolled. Suitably, the control means comprises a current pulsegenerator for applying control current pulses to the third coil means.Conveniently the pulse generator is able to control the amplitude and/orduration of the pulses to control the magnetic field applied by thethird coil means.

Conveniently the first coil means can be made of any kind (high orlow-temperature SC) and of any form (wires, bulk) of superconductingmaterial. Preferably, however, the first coil means is made of a bulkceramic high-temperature superconducting material, such as YBCO orBSCCO, preferably arranged in a single turn. Preferably thesuperconducting material should be anisotropic such that the maximumallowed current in the direction of the axis (the “c-axis”) of the firstcoil means is much less than the critical current in the plane (the “a-bplane”) perpendicular to the c-axis. The reason for this is that it isnecessary to make the superconducting first coil means normallyconducting (i.e. non-superconducting) in order to charge and dischargethe SMES. This is achieved by applying a field-pulse in the a-b planewhich is sufficiently large such that the super-conducting material istransferred to its normal conducting state. The more anisotropic thesuperconducting material is the smaller amplitude of the pulse that isrequired.

Suitably the cooling means comprises a cryogenic container, e.g. adewar, in which the first coil means is situated. The second coil meansis preferably arranged outside the cryogenic container. By suitableshaping of the cryogenic container, the third coil means may also besituated outside the cryogenic container. For example, in the case ofthe first coil means being in the form of a cylinder, the cryogeniccontainer may have an annular form with the third coil means surroundinga part of the annular cryogenic container.

Preferably the third coil means is arranged to supply a magnetic fieldin a plane substantially perpendicular to the main (cylindrical) axis ofthe first coil means.

Preferably the first and second coils are coaxial with each other.

According to another aspect of the present invention there is provided amethod of inputting energy to and/or outputting energy from a first coilmeans made of superconducting material and cooled to superconductingtemperatures, comprising inductively coupling second coil means to thefirst coil means, wherein the first coil means is arranged in a closedelectrical circuit with no mechanical connections thereto for inputtingor outputting energy to the first coil means and wherein the first coilmeans is rendered non-superconducting or superconducting by theapplication or removal of a magnetic field via a third coil means.

Preferably the magnitude of the magnetic field applied by the third coilmeans is controllable. By applying a controllable current, e.g. viacurrent pulse control, the first coil means can be partially quenched sothat energy can be partially extracted from or supplied to the firstcoil means.

According to a still further aspect of the present invention there isprovided a power supply system including a magnetic energy storagedevice according to said one aspect of the present invention. The powersupply system may be able to provide energy storage, power quality, peakload security and/or power supply security. Power quality, peak loadsand power supply security may be specified closely and numerically intypical contracts for delivery and maintenance of electrical power by apower supply system.

DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with particular reference to the accompanying drawings, in which:

FIG. 1 is a schematic partly sectioned perspective view of oneembodiment of a SMES device according to the invention;

FIG. 2 is a schematic circuit diagram illustrating the principle ofoperation of the SMES of FIG. 1;

FIG. 2 a is a modification of the schematic circuit diagram shown inFIG. 2;

FIG. 3 is a schematic view of another embodiment of a SMES deviceaccording to the invention; and

FIG. 4 is a block diagram of a power supply system according to theinvention.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows one embodiment of a SMES according to the invention andgenerally designated by the reference numeral 1. The SMES 1 comprises aone turn first coil in the form of a cylinder 2 and made ofsuperconducting material, such as YBCO having a T_(c)=92 K. The cylinder2 forms a closed electric circuit and has no mechanical or physicalconnections connected thereto for supplying energy to or extractingenergy from the coil. The cylinder 2 is cooled to superconductingtemperatures by cryogenic cooling means. In the embodiment shown, thecylinder 2 is received in a container 3, e.g. a conventional dewar,containing liquid nitrogen at a temperature of 77 K. Alternatively asuitable enclosure means for containment and circulation of helium (orother coolant) in a gas, liquid or gas/liquid mixture may be used.Cooling down to between 40 K and 20 K or less in this manner provides ahigher magnetic flux density in high temperature superconductors such asYBCO or BSCCO.

A second coil 4 for outputting stored energy from the cylinder 2 iswound outside the container 3, is coaxial with the cylinder 2 (in thedirection of the c-axis (see FIG. 1) and is inductively coupled with thecylinder 2. A load (not shown) is connected to the coil 4. In practicethe coaxially arranged coil 4 is also used for inputting energy to thesuperconducting cylinder 2.

A third coil 5 is wound around the cylinder 2 or a portion thereof. Asshown the coil 5 is positioned inside the container 3 but, in practice,the container 3 would be suitably shaped, e.g. in the form of an annulartrough or the like, to enable the coil to be wound around the container2 and to be positioned outside the container 3. Current control means(not shown) are provided for supplying current pulses to the third coil5.

In use energy in the form of magnetic field is stored in thesuperconducting cylinder 2. Energy is extracted by crossing the knownconducting-to-superconducting phase transition line, which is a functionof H (magnetic H field) and j (current density) for a given temperaturefor the superconducting material. For an anisotropic superconductingmaterial these values depend on the direction of the applied field orcurrent. In practice, the superconducting material is made to go normal(i.e. non-superconducting) by applying a magnetic field in the a-b plane(see FIG. 1). The energy stored in the system starts to dissipate in theload resistance connected to the coil 4.

The operation of the SMES device 1 is illustrated schematically in FIG.2. The one-turn coil in the form of cylinder 2 is arranged in a closedcircuit. The “switching” function of the third coil 5 is illustratedsymbolically in FIG. 2. In particular, when the superconducting materialof the cylinder 2 is superconducting, switch 7 can be considered closed.When current is applied to the coil 5, however, and a magnetic field isapplied in the a-b plane, the superconducting material is renderednormal or non-superconducting. In this case, the switch 7 can beconsidered open so that resistance 8 is introduced into the circuit. Asmentioned above, this is only a symbolic representation, FIG. 2 showingan equivalent circuit for the quenching coil 5.

The coil 5 acts as a means for controllably quenching thesuperconducting material of the cylinder 2 for switching thesuperconducting material between its superconducting andnon-superconducting states. The preferred method of controlling thecurrent supplied to the coil 5, and thus controlling the switching ofthe superconducting material, is to use a current pulse generator (notshown). Characteristics of the current pulses, e.g. amplitude and/orlength of the current pulses, pulse rise time, aggregate current value,pulse decay time, can be controlled to provide the desired degree ofquenching. In this manner it is possible to extract only a part of theenergy stored, i.e. a phenomenon that can be referred to as “partialquenching”. A well-chosen combination of amplitude and pulse length canrelease a desired amount of energy.

As mentioned above charging of the SMES device 1 can be achieved in amanner that in principle is the reverse to the procedure described abovefor extraction of energy. In this case, energy may be supplied to thecylinder 2 via the induction from the coaxial coil 4. A current isapplied to coil 5 that causes quenching of the coil 2 so rendering thelatter normally conducting (as opposed to superconducting). When currentis supplied through the coil 4, a current is induced in the cylinder 2which is, however, limited and reduced by the electrical resistance 8present. When the quenching current is removed from the coil 5, thecylinder 2 returns to its superconducting condition. The removal ofcurrent from the coil 4 induces a current in the superconducting coil 2.In an alternative method of charging the SMES device 1, a current isapplied to the coil 4 inducing an opposing current in the cylinder 2. Aquenching current is applied to the third coil 5 so that the currentinduced in the cylinder 2 dies away. Current continues to flow in coil4. When the quenching current is removed from coil 5, the cylinder 2becomes superconducting again. When the current is removed from the coil4, a current is then induced in the cylinder 2.

The present invention makes use of the transfer of energy between asuperconducting (cryogenic) first coil means to a normal conductingsecond coil means at elevated, e.g. room temperature, without the use ofan oscillating circuit. The required time varying current is insteadobtained by making the superconducting material normal-conducting with alarge enough magnetic field from the third coil means perpendicular tothe other two coil means. As long as this quench-field is applied thecurrent in the first coil means decays. According to Faraday's law ofinduction the secondary coil tries to oppose this change by inducing acurrent in the same direction. This current is used to provide a loadwith power. In practice a capacitor 12 (see FIG. 2 a) is charged to adesired voltage which, in turn, is discharged in a controlled way over aload. (If the load requires an ac-current there must be a dc-acconverter between the capacitor and the load.) Symbolic switch means 6is used to direct current flow in the required direction. For extractionof energy, the switch means, e.g. diode means, prevents current fromflowing back towards the coil 4. For injection of energy, the switchmeans is effectively closed.

The invention described herein makes it possible to partially or fullydischarge (or charge) a closed-loop superconductor by inductivecouplings. This opens the possibility to isolate the cryogenic partsfrom other parts of the device or system that can be at elevated, e.g.room temperature. Thus heat leakage into the cryogenic parts can beminimised. Furthermore, since no mechanical connections are made to thecylinder 2, resistive losses are minimised. The problem of the prior artwith switching current from the superconducting coil to a normalconducting circuit is avoided by having no physical or mechanicalcontact between the systems. By partial quenching of the superconductingmaterial, energy can be supplied to or extracted from thesuperconducting coil in a controlled manner.

FIG. 3 illustrates another, presently less preferred embodiment of theinvention in which a high permeability magnetic core 10 is providedhaving at least one air gap 11 for increasing the energy storagecapacity. In this embodiment, a second coil 4′ is not wound directlyoutside the superconducting first coil means or cylinder 2′ but isinstead wound on the core 10. The coil 4′ is preferably situated aroundthe air-gap(s) 11 in the core.

It is possible to connect a capacitor to the discharge circuit whichmakes it possible to distribute the energy in a more controlled manner.The energy is stored long-term in the superconducting cylinder of theSMES and, when needed, it is extracted little by little. In thisscenario the capacitor works as an intermediate storage device. Anapplication would be to use it as an energy quality booster because ofthe short access time. This means that when a voltage drop is detectedin the electricity net the SMES is quickly switched on to ensure aconstant voltage level.

FIG. 4 illustrates a typical control system using a number of inputs.The control information may include actual current density in thecylinder, actual voltage across the capacitor, a feedback signal fromthe load, and the actuators would be the pulse length and the quenchcurrent in the pulse generator.

Although the superconducting coil (cylinder 2) is preferably ofcylindrical form, it may take other forms providing a closed electricalcircuit and being of a fixed shape. The superconducting material, e.g.ceramic material, may, for example, be pressed and sintered into adesired shape, such as a toroidal shape, which is not plasticallydeformable.

The use of a power supply device according to the invention enables apower supply system of a specified electrical power quality or powerload to be provided. By using a SMES device an economic advantage can beobtained since more stringent power quality/load matching specificationscan be achieved without the use of additional generators, reactors,etc.. The invention has application in maintaining power quality againstshort term power or voltage reductions, in serving as a storage systemin order to smooth peak loads, in providing an “uninterruptible” powersupply in a grid or even in a supply system for an industrial processwhere power disturbances are very expensive and damaging, such as, forexample, a paper mill or a steel mill. A power supply system accordingto the invention may also have application as an energy storage devicein vehicles, including ships, aeroplanes and other types of vehiclessuch as electric and hybrid electric/combustion powered cars, trucks andbuses.

1. A superconducting magnetic energy storage (SMES) device comprising:first coil means made of superconducting material; cooling means forcooling the first coil means to superconducting temperatures; secondcoil means inductively coupled to the first coil means for inputtingenergy to, and/or outputting energy from, the first coil means; andswitching means for switching the first coil means between asuperconducting condition and a non-superconducting condition, whereinthe first coil means is arranged as a closed loop electric circuithaving no connecting means mechanically connected thereto for inputtingor outputting energy thereto, and wherein the switching means comprisesthird coil means for application or removal of a magnetic field forswitching the first coil means between its non-superconducting andsuperconducting conditions.
 2. A SMES device according to claim 1,wherein the first coil means comprises a single turn.
 3. A SMES deviceaccording to claim 2, wherein the first coil means comprises a cylinder.4. A SMES device according to claim 2, wherein the first coil means isof toroidal form.
 5. A SMES device according to claim 3, wherein thefirst coil means comprises ceramic material and has a fixed shape whichis not plastically deformable.
 6. A SMES device according to claim 1,wherein the switching means further comprises control means forcontrolling the current supplied to the third coil means.
 7. A SMESdevice according to claim 6, wherein the control means comprises acurrent pulse generator for applying control current pulses to the thirdcoil means.
 8. A SMES device according to claim 7, wherein the pulsegenerator has means for controlling amplitude and/or duration of thepulses to control the magnetic field applied by the third coil means. 9.A SMES device according to claim 1, wherein the first coil meanscomprises high temperature superconducting material.
 10. A SMES deviceaccording to claim 1, wherein the first coil means comprises lowtemperature superconducting material.
 11. A SMES device according toclaim 1, wherein the superconducting material is anisotropic.
 12. A SMESdevice according to claim 1, wherein the cooling means comprises acryogenic container, in which the first coil means is situated.
 13. ASMES device according to claim 12, wherein the second coil means isarranged outside the cryogenic container.
 14. A SMES device according toclaim 12, wherein the third coil means is situated outside the cryogeniccontainer.
 15. A SMES device according to claim 1, wherein the thirdcoil means is arranged to supply a magnetic field in a planesubstantially perpendicular to a cylindrical axis of the first coilmeans.
 16. A SMES device according to claim 1, wherein the first andsecond coils are coaxial with each other.
 17. A power supply systemincluding a magnetic energy storage device according to claim
 1. 18. Amethod of inputting energy to and/or outputting energy from first coilmeans made of superconducting material and cooled to superconductingtemperatures, comprising: inductively coupling second coil means to thefirst coil means, wherein the first coil means is arranged in a closedelectrical circuit with no mechanical connections thereto for inputtingor outputting energy to the first coil means, and wherein the first coilmeans is rendered non-superconducting or superconducting by applicationor removal of a magnetic field by third coil means.
 19. A methodaccording to claim 18, wherein a magnitude of the magnetic field appliedby the third coil mean is controllable.
 20. A method according to claim18, wherein a duration of the magnetic field applied by the third coilmeans is controllable.
 21. A method according to claim 18, wherein themagnetic field applied by the third coil means is pulsed and a pulselength of the applied magnetic field is controlled.
 22. A methodaccording to claim 19, wherein a field applied by the third coil meansis controlled by applying controlled current pulses to the third coilmeans.
 23. A method according to claim 19, wherein characteristics ofthe current pulses are controlled.